NEW DELHISonia Gandhi, the leader of India's main opposition party, has undergone shoulder surgery at a New Delhi hospital after suffering a fall on a campaign stop this week, senior Congress party officials said on Friday.

Gandhi, 69, dislocated her left shoulder when she fainted on Tuesday while launching the party's campaign for next year's Uttar Pradesh state election in the holy city of Varanasi, Prime Minister Narendra Modi's parliamentary constituency.

Gandhi suffered acute dehydration while travelling in an open vehicle for four hours through large crowds.

"She had dislocated her shoulder after a fall and is now under observation for 48 hours," said a senior party official, who did not wish to be named as he was not authorised to speak to the media.

The Italian-born politician, who led her party to two successive terms governing the world's largest democracy, has played a slightly reduced public role since being treated abroad for an unknown illness in 2011.

Despite persistent speculation, she has yet to hand over the party leadership to Rahul Gandhi, her 46-year-old son and the great grandson of India's first prime minister Jawaharlal Nehru.